The present invention relates to a system for controlling injection timing of fuel for an internal combustion engine with a direct fuel injection system, and more particularly to the system for promoting stratified combustion.
In the direct fuel injection system, a rich air-fuel mixture is injected from an injector near a spark plug while dispersion of the fuel is suppressed. The mixture is ignited with the spark plug and continuously burned with flame propagation to achieve the stratified combustion.
In order to improve scavenging efficiency in a light engine load range of a two-cycle engine and to prevent the fuel from escaping through an exhaust port, the direct fuel injection system has been proposed. In the system, air is only induced into the cylinder by a scavenge pump through a scavenge port and the fuel is directly injected to the cylinder by a fuel injector after closing the scavenge port. In such a system, the fuel injector and a spark plug are disposed in a combustion chamber of the cylinder.
In a four-cycle engine, the engine load is controlled by controlling the quantity of the fuel from the injector, thereby reducing throttle loss of induced air. Thus, the air-fuel ratio becomes lean to improve thermal efficiency and fuel consumption. Therefore, it is desirable to achieve the strafified combustion in a wide operational range of the engine. However, if the mixture is burned in a stratified mode in a heavy engine load range, the air can not be sufficiently used and hence large engine power can not be produced . Therefore, it is necessary to immediately change the combustion to a homogeneous mixture combustion.
In order to ensure a smooth transfer of the combustion between the stratified charge and the homogeneous mixture, it is necessary to properly determine arrangements of the injector, the spark plug and the gap of the plug, flow of injected fuel, fuel injection rate, and the fuel injection timing corresponding to the ignition timing. The fuel is injected from the injector in a period from closing the exhaust valve to ignition. The period is long in the four-cycle engine compared with that of the two-cycle engine. In any engine, it is necessary to inject the fuel immediately before the ignition in order to achieve the stratified combustion. Since the ignition timing and the fuel injection quantity vary in accordance with engine operating conditions, it is desirable to properly control the starting time and the finishing time of the fuel injection.
Japanese Patent Application Laid-Open 62-32214 discloses such a two-cycle engine. The two-cycle engine has a spherical cavity formed in a roof of a combustion chamber, positioned eccentrically with respect to an axis of the cylinder The cavity is effective to promote turbulence of fresh air in a vertical direction to form a vertical swirl. Injected fuel is mixed with turbulent air, which is carried toward a spark plug. A fuel injector and a spark plug are provided on a top of the cavity.
In the system, it is evident by measuring flow in the cavity with a laser doppler anemometer that a cycle of the swirl greatly fluctuates in accordance with engine operating conditions. Accordingly it is difficult to stably ignite the fuel in any engine operating condition. In a heavy engine load range, the fuel is not equally dispersed due to the swirl so that the homogeneous mixture combustion is not obtained. Consequently, large engine power in the heavy load range can not be expected.
Japanese Patent Application Laid-Open 60-501963 discloses a direct fuel injection system where the fuel is injected in the cylinder at 5 to 7 kg/cm.sup.2 of injection pressure.
In the direct fuel injection system, the fuel is injected after the exhaust port is closed in a compression stroke Since the pressure in the cylinder is high in a late stage of the compression stroke, the fuel can not be injected into the cylinder. Consequently, it is difficult to set the fuel injection time at a proper time in accordance with the engine operating conditions. As a result, the ignition and fuel injection are inevitably set at earlier times. In particular, the fuel injection timing for providing the stratified charge is not properly controlled.